Manufacturing operations are increasingly faced with the difficult task of reducing emissions of volatile hazardous air pollutants (VHAPs) and volatile organic compounds (VOCs) from their production facilities in order to achieve compliance with stringent regulations promulgated by the United States Environmental Protection Agency. The costs associated with direct treatment of exhaust streams by conventional technologies (such as thermal oxidation, catalytic oxidation, carbon absorption, or liquid absorption) in many cases make conventional technologies unattractive. Adsorption of VHAPs/VOCs with activated carbon or zeolite may provide some cost savings, but these technologies require that the adsorption material be regenerated and/or disposed of, raising costs to approximately the same levels as thermal or catalytic oxidation. The technology disclosed herein will be more cost-effective than other technologies, because of reduced treatment volume and low operating costs.
Recently, biofiltration has emerged as an alternative to conventional VHAP/VOC treatment technologies. In general, biotreatment provides a cost-effective and efficient means of destroying organic pollutants. There are no fuel requirements as in thermal/catalytic oxidation units, and there are no hazardous discharges from the reactor system. Pollutants are consumed by resident microorganisms as food for growth and energy. However, biofilters suffer from the same difficulties as other control technologies when considered for application to most manufacturing processes. A typical biofilter requires a bed volume roughly equal to the air flow which it must treat. Therefore, a biofilter which will process 300,000 CFM air requires about a 300,000 ft.sup.3 bed volume. The tremendous size of these units places capital costs in the three to seven million dollar range with estimated annual expenses between $650,000 and $2.5 million.
Biofilter size requirements arise from the fact that they directly treat air emissions. This means that units are sized for air flow, not pollutant loading. Also, successful application of a biofilter requires that pollutants have sufficient opportunity to be absorbed/adsorbed by the packing material. Because the local bed environment is aqueous, water soluble compounds (e.g., alcohols, ketones) are removed from an air stream more efficiently than non or slightly soluble compounds (e.g., toluene, xylene). This limitation has a negative impact on the efficiency of treating aromatic pollutants.
Methods for extracting pollutants from waste water and gases for biodegradation are known in the art.
Brookes et al., Biotechnol. Prog. 10, 65-75 (1994), discloses an extractive membrane bioreactor system for treating waste water containing chloroanilines. The extractor unit is comprised of a shell containing silicone rubber tubes, which act as the extractive membrane. The outflow from the extraction unit is directed into a fermentor that serves as the bioreactor. A stream of water containing chloroanilines is put through the membrane extraction unit, where the chloroanilines are extracted across the silicone rubber tubes into an aqueous biomedium, which is then fed into the bioreactor for degradation.
U.S. Pat. No. 4,988,443 to Michaels et al. teaches methods and apparatus for continuously removing organic toxicants or other oleophilic solutes from waste water. The process employs hydrophobic hollow fiber membranes in a flow reactor to remove organic toxicants from a liquid stream. The organic toxicants in the liquid process stream are selectively transported across the hollow fiber membrane, where they are degraded by resident microorganisms into water soluble metabolites.
Livingston, Biotechnol. Bioeng. 41, 915-26 (1993), discloses a membrane bioreactor unit for removing and biodegrading phenol in waste water. The membrane bioreactor unit is a glass tube containing an aqueous biological growth medium. A silicone rubber tube wrapped around a stainless-steel mesh support is placed therein. The silicone rubber tube acts as a membrane to remove phenol from waste water. Waste water containing phenol is pumped through the silicone rubber coil. Phenol diffuses across the membrane coil, into the aqueous biological growth medium, where degradation occurs by microorganisms residing in the bulk fluid and in a biofilm on the outside of the silicone rubber membrane. In a companion article, Livingston, Biotechnol. Bioeng. 41, 927-36 (1993), disclosed the use of the same bioreactor unit to remove and biodegrade 3-chloronitrobenzene from waste water.
Choi et al., Biotechnol. Bioeng. 40, 1403-1411 (1992), teach a method of biodegrading aromatic solvents in waste water. The biotreatment unit is comprised of a silicone tube immersed in an aerated bioreactor. Liquid toluene is circulated within the tube. The toluene diffuses out of the tube into the aqueous culture medium where it is aerobically degraded by microorganisms.
Deshusses et al., Biotechnol. Bioeng. 49, 587-598 (1996), disclose the use of a biofilter to remove methyl ethyl ketone (MEK) and methyl isobutyl ketone (MIBK) vapors from air. The microorganisms in the biofilter are immobilized to an inert material comprising compost and polystyrene spheres packed into an acrylic glass column. Compressed humidified air streams are sparged with either MEK or MIBK and then passed through the biofilter column, where resident microorganisms degrade the MEK/MIBK in the humidified air stream.
Deshusses et al. further examine the transient-state performance of the biofilter in response to perturbations. They observed that the biofilter experiences a 4.5 day acclimation period after initial start-up before attaining full removal efficiency. In addition, these investigators reported that step changes in MEK or MIBK inlet concentrations or gas flow rate cause disruptions in the system resulting in a 2-5 hour lag before a new steady-state is achieved. Experiments in which the biofilter was pulsed with high concentrations of MEK or MIBK resulted in transient "break through" of pollutant into the effluent, indicating the biofilter is unable to respond to rapid increases in pollutant concentrations. Finally, the effects of starvation on biofilter performance was assessed. After a 5-day starvation period, the biofilter experienced a performance lag of several hours after the system was restarted.
Freitas dos Santos et al., Biotechnol. Prog 11, 194-201 (1995), teach a method for removing dichloroethane (DCE) from a gas stream using a membrane bioreactor system. DCE is extracted across a hydrophobic dense phase silicone rubber membrane where it is then degraded by a biofilm attached to the membrane. These investigators note that biofilm growth on the membrane limits removal efficiency; the biofilm reduces the cross-sectional area for fluid flow, which results in a reduced mass transfer of DCE across the membrane.
In view of the foregoing there is a need for improved systems for biotreating volatile organic pollutants in gas exhaust streams.